Continue with physiotherapy. Continue muscle relaxant medications for a week more. If the weakness increases, get MRI done and referr back with reports.

You may have a traumatic disc, the weakness needs to be examined clinically and an MRI needs to be done. The weakness corroborated with the MRI findings will warrant further treatment, conservative or otherwise. I feel you should consult a spine surgeon.

I agree with Dr Darekars assessment of nerve compression as the most likely cause of the symptoms. Even though the weakness has presented a few weeks after the injury, ruling out a disc prolapse is important. In the initial stages after the injury, the spasm of the neck muscles keep the area protected and sometimes the clinical picture becomes more obvious only when the spasm is relieved. I also agree that a good clinical and MRI are the next appropriate steps.